Currently, the commonly used positive electrode material for high energy density lithium ion battery is LiCoO2. Its energy density has reached its limit under the limitation of compacted density, maximum voltage, etc. Further, the battery polarization is great under this energy density thus it is difficult to meet the requirements of high output power and long working life. Compared with LiCoO2, the layered lithium-nickel composite oxide positive electrode material LiNixM1-xO2, wherein M is one or two selected from the group consisting of Co, Mn, Al, Cr, Mg, Cu, Ti, Zn, Zr and V and wherein 0.5≦x<1, draws more and more attention in the field of lithium battery due to its advantages of high discharge capacity (170-230 mAh/g) and low cost.
In LiNixM1-xO2, Li atom is located at the position 3a, the transition metal atom is located at position 3b, and O atom is located at the position 6c of the M′O6 octahedron (M′=Ni, Co, Mn, Al or Cr). Since the radius of Li+ (0.76 Å) is very close to that of Ni2+ (0.69 Å), during the high temperature sintering process, trace amount of Li volatilize, and part of Ni occupies the 3a position of Li in the crystal structure, forming a tiny structural subsidence area and lead to the lithium/nickel mixed arrangement. Lithium/nickel mixed arrangement results in escape of internal structure active oxygen and increase of the number of free lithium ion. The escaped active oxygen further contact and react with CO2 and H2O in the air to form CO32− and OH−. The resulted CO32− and OH− continue to react with the active lithium ion to generate lithium soluble salts such as Li2CO3 and LiOH, which adheres to the active substance surface, thereby increase pH of the material. High pH will have serious impact on performance of the material:                (1) the material is highly hygroscopic to deteriorate, thus having poor compatibility with PVDF binder, poor dispersion and stability of slurry, easy gumming and low product yield.        (2) the material will react with aluminum foil to generate flocculent precipitate Al (OH)3, which will hinder the transfer of lithium ions to some extent, thereby affecting the battery capacity retention ratio and the rate capability.        (3) pH increases and the moisture content at the material surface increase correspondingly, therefore in the battery high temperature storage, LiPF6 reacts with the water introduced by nickel-based material to generate HF and HF further reacts with impurity Li2CO3 and LiOH to generate CO2 gas and H2O. H2O, as an initiator further catalyze decomposition of LiPF6 thereby seriously reducing the battery performance and ultimately severely affecting the high-temperature storage performance and circulation stability of the lithium ion batteries using nickel-based material as a positive electrode material. Particularly, with respect to the lithium ion battery using aluminum-plastic composite film as packaging, the shell is fairly soft and gas production will lead to battery rapid expansion and deformation, thus producing serious security risks and limiting its use.        
Chinese invention patent CN101572308A discloses a method for improving the overall performance of a lithium ion electrode material. The method uses a volatile organic solvent to stir and dry the electrode material, thus effectively preventing combination of the electrode material and water molecules to avoid gumming problems and improve processing performance of the electrode material. However, this method cannot remove the impurities Li2CO3 and LiOH at the surface of the electrode, nor decrease pH of the material, thus the high temperature storage performance and circulation stability of the battery are not essentially improved.
Chinese invention patent CN102683672A disclosed a method for lowering pH of ternary materials, wherein the ternary materials are rinsed and filtrated with deionized water or a solution containing HCO3− and placed in a muffle furnace for two-stage sintering, thereby effectively reducing the pH value of the ternary materials and contributing to improve the electrochemical properties of the ternary materials. However, the method is comparatively complicated as twice high temperature sintering processes are required. It will not only prolong the production cycle and increase energy consumption, but also will produce more industrial waste water. Studies have shown that after the layered nickel-based positive electrode material LiNi0.8Co0.1Mn0.1O2 is washed with water, the structural stability and circulation stability in the electrolyte of the material has been significantly improved, while the first discharge specific capacity of the material is significantly lowered and largely affected by the temperature of the second sintering. In addition, when the material washed with water is exposed to air, it become easier to absorb moisture, thereby developing higher requirement for the material packaging, transport, storage and usage environment.